The present invention relates to an improved structure and a method for fabrication of a charge coupled device (hereinafter referred to as a CCD) image sensor for reducing a smear noise.
In general, a CCD image sensor has a structure that a p-type well is formed on an n-type substrate and an n-type photo diode and an n-type vertical charge coupled device (hereinafter referred to as a VCCD) are formed on a p-type well at a designated interval and a transfer gate for connecting said n-type photo diode and said n-type VCCD is formed over the surface of the region between them and an n-type buried charge coupled device (hereinafter referred to as a BCCD).
In said CCD image sensor, an n-type VCCD may be substituted for the BCCD or a surface charge coupled device (hereinafter referred to as a SCCD). Now, the SSC is seldom used.
In a CCD image sensor of the aforesaid conventional structure, to reduce the blooming phenomenon appearing on screen, an anti-blooming bias for controlling an over flow drain (hereinafter referred to as OFD) voltage is ordinary applied to the region under said n-type photo diode.
Thus, a designated potential barrier is formed to prevent overflow of a signal stored in potential well.
Previously known methods for controlling an OFD voltage are a horizontal over flow drain (hereinafter referred to as HOFD) method and a vertical over flow drain (hereinafter referred to as VOFD) method.
However, because the HOFD method is a clocking method, the VCCDs corresponding to each photo diode should be arranged in a line. Accordingly the open area of a photo diode is relatively reduced and the fill factor is decreased so that the sensitivity of the CCD image sensor is lowered.
Currently, the VOFD method is used as the method for controlling an OFD voltage. In said VOFD method, a shallow p-type well of a fitting depth is formed under the photo diode region and a deep p-type well of a fitting depth is formed under the other region by implanting ions twice so that an adequate anti-blooming bias is applied.
A structure of a CCD image sensor made by a VOFD method is shown in FIG. 1(a). An n-type epitaxial layer 2 is formed on an n-type subtrate 1 and a shallow p-type well 3 and a deep p-type well 4 is formed on an n-type epitaxial layer 2 by implanting ions twice. An n-type photo diode 5 and an n-type BCCD 6 are formed in the upper region of said shallow p-type well 3 and said deep p-type well 4 respectively. Subsequently, a transfer poly gate 7 for connecting said n-type photo diode 5 and said n-type BCCD and a poly gate electrode 7a for applying a clock signal to said n-type BCCD 6 are formed over the surface of the region between them and said n-type BCCD 6.
As shown in FIG. 1(a), if light is radiated on an n-type photo diode 5 and a signal charge is generated under an n-type photo diode, a signal charge is shifted to an n-type BCCD 6 by a high level signal which is applied to a transfer poly gate 7 and stored under an n-type BCCD 6. At this time, a signal charge is shifted to a BCCD by conventional clocking of a CCD.
FIG. 1(b) shows an electric potential distribution taken along the line a--a' of FIG. 1(a).
However, at the same time, a signal charge generated under said n-type photo diode 5 is drifting between a deep p-type well 4 and an n-type BCCD 6 or is discharged to an n-type substrate 1 and this charge causes the smear phenomenon.
Furthermore, when a shutter voltage which is on the order of 30 V through 40 V is applied to an n-type subtrate 1, this smear charge is discharged to an n-type subtrate 1 by the shutter voltage and the smear phenomenon can be further increased. This is because this shutter voltage is very intense.
In the prior art, to prevent the smear phenomenon a p.sup.+ -type blocking p-type layer (hereinafter referred to as a BPL) has been formed in a designated region between said n-type BCCD 6 and said deep p-type well 4 by implanting p ions with high energy.
The process for fabrication of a CCD image sensor using p.sup.+ -type BPL according to the prior art will be described with reference to the accompanying FIG. 2(a) through FIG. 2(f).
First, as shown in FIG. 2(a), an n-type epitaxial layer 2 is formed on an n-type substrate 1 and as shown in FIG. 2(b) p-type ions are implanted onto an n-type epitaxial layer 2 twice to control an OFD voltage and as shown in FIG. 2(c) a shallow p-type well 3 and a deep p-type well 4 of a designated depth are formed by heat treatment which is used for diffusing the implanted p-type ions.
Next, as shown in FIG. 2(d), p.sup.+ -type BPL 8 is formed in a designated region of said deep n-type well 4 by implanting p-type ions by means of ion implantation apparatus of high energy (about 600 KeV). Thereafter the p.sup.+ -type BPL 8 prevents the smear phenomenon that signal charge stored in the BCCD is discharged to a substrate by a shutter voltage of a substrate and that a signal charge generated in photo diode is not transfer to said BCCD but is discharged to a substrate.
As shown in FIG. 2(e), an n-type photo diode 5 is formed in a designated portion of the upper region of said shallow p-type well 3 by implanting n-type ions and an n-type BCCD 6 is formed in a deep p-type well 4 which is in the upper region of said p.sup.+ -type BPL 8 by implanting n-type ions.
At this time, a conventional p.sup.+ -type thin film 9 is formed in the surface of an n-type photo diode 5.
Next, as shown in FIG. 2(f) a transfer poly gate 7 for connecting said n-type BCCD 6 and a poly gate electrode 7a for applying a clock signal to said n-type BCCD 6 are formed over the surface of the region between them and said N-type BCCD 6. In this process, poly silicon is used for a transfer gate. Metal such as Al instead of poly silicon can be used for it but it is almost out of use due to its undesirable transfer characteristics.
FIG. 3(a) shows a reference diagram to explain the operation of a CCD image sensor fabricated by the above-mentioned process and the operation of a CCD image sensor will be described with reference FIG. 3(a).
Now, if light (.lambda.) is radiated on an n-type photo diode 5, a signal charge is generated in the light signal charge output region 0 which is disposed between said n-type photo diode 5 and a shallow p-type well 3. When a driving signal of high level is applied to a transfer poly gate 7, this signal charge is stored in the signal charge store region Q adjacent to an n-type BCCD 6 through the signal charge transfer channel region P which is disposed between an n-type photo diode 5 and an n-type BCCD 6.
Next, a signal charge stored in said signal charge store region Q is shifted to a horizontal charge coupled device (hereinafter referred to as a HCCD) (not shown) by the conventional clocking operation.
At this time, if a signal charge generated in the light signal charge output region 0 is not passed to the signal charge transfer channel region P but is discharged to the smear signal output region R which is disposed between a deep p-type well 4 and a p.sup.+ -type BPL 8, the smear phenomenon is generated on screen of a CCD image sensor.
But, as shown in FIG. 3(b) which shows an electric potential distribution taken along the line b--b' of FIG. 3(a), it is hard to discharge a signal charge to the smear signal output region R with high potential barrier of said p.sup.+ -type BPL 8 and therefore the smear phenomenon is reduced.
Actually, a signal charge which is drifting in an n-type BCCD 6 rather than a smear signal which is discharged to an n-type substrate 1 is further discussed.
In said structure of a CCD image sensor of prior art, a shallow p-type well and a deep p-type well is formed in a flat shape by implanting ions twice to apply an anti-blooming bias but the former can also be formed in a heart.
Now, by forming said p-type well in a flat shape and controlling the impurity concentration of the region under a photo diode and a BCCD when implanting ions, a structure and a method for fabrication of a CCD image sensor which resists the smear phenomenon and controls the OFD voltage has been studied. But they not yet in use due to difficulties with the ion implantation process.
Prior art structures and a methods for fabrication of a CCD image sensor as shown in FIG. 2 have disadvantages as follows.
First, because ion implantion apparatus for forming p.sup.+ -type BPL is very expensive and the usage of it is restricted, it is not of practical use.
Second, because p-type ions are implanted using high energy of about 600 KeV, defects are caused in the substrate by the implanting ions. Accordingly, because noise may be generated by defects in CCD image sensors of the prior art, the smear phenomenon can be reduced but a high level of process technology to form the p.sup.+ -type BPL is demanded.